Method for manufacturing a high-performance package for monolithic microwave integrated circuits

ABSTRACT

The present invention incorporates a specially-designed low-pass filter into the feedthrough of a monolithic microwave integrated circuit (MMIC) to provide compensation for discrepancies from the impedance required for an MMIC package to be matched to a transmission line. The compensation allows all parameters to be adjusted and the complete filter to be printed. 
     The main features of the invention include minimum width for the under-wall conductor and brazing metallization, two open-circuited stubs at the package-die interface for wire bonding ease, and metal-filled vias connecting the ground plane base and the lid sealing ring to bring the lid sealing ring to RF ground. 
     The present invention also includes a method for designing the desired feedthrough the obviates the need for scale-model feedthrough design before printing, a prior art method that requires precision in the scale model. An electrical model of a feedthrough is first derived. The electrical model is then adjusted according to the parameters desired for a new, compensated feedthrough using any known method, including software such as touchstone. Finally, the new feedthrough is fabricated based upon the adjusted electrical model.

This application is a division of application Ser. No. 07/251,446, filedSept. 30, 1988, now U.S. Pat. No. 4,901,041.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to packaging for integratedcircuits, and, more specifically, to high-performance packaging formonolithic microwave integrated circuits (MMICs) operable at frequenciesin the 20 GHz range.

2. Description of the Related Art

Packaging techniques known to the prior art have been well documented.Known manufacturing techniques include cofired ceramic enclosures usingthick-film metallization, glass-sealed ceramic enclosures usingthin-film metallization, metal enclosures having ceramic feedthroughs,and metal enclosures having glass feedthroughs.

Since ceramic-package manufacture is a batch process, it has a distinctadvantage over some of the other technologies available. If the yield ofthe process is high, ceramic packages should be low cost in reasonablevolume.

Of the few cofired ceramic packages generally available for MMICs andpower FETs, the main contributor to poor microwave performance is thelead feedthrough. In typical state-of-the-art designs, discontinuitiesexist due to the lead attachment, to passage of the conductor into andout from the ceramic wall, to changes in the conductor width, and tocoupling of RF signals to the lid and lid sealing ring. Thesediscontinuities introduce higher-order modes and reflections as a resultof impedance mismatch, and contribute to overall poor feedthroughperformance, especially at frequencies above 20 GHz.

An MMIC package capable of good performance in the 20 GHz range shouldhave low insertion loss and voltage standing wave ratio (VSWR) per leadfeedthrough, good isolation between leads, microstrip compatibility, andan electrical design approach extendable to higher frequencies. Thepresent invention fulfills all of these goals in a package that iscascadable, bondable, and capable of extensive application throughoutthe entire MMIC industry, including military applications.

SUMMARY OF THE INVENTION

The present invention incorporates a specially-designed low-pass filterinto the feedthrough itself to provide compensation for discrepanciesfrom the impedance required for an MMIC package to be matched to atransmission line (typically 50 Ohms). The compensation used is similarto LC techniques known to the prior art, but allows all parameters to beadjusted and the complete filter to be printed.

The main features of the present compensated feedthrough include minimumwidth for the under-wall conductor and brazing metallization, twoopen-circuited stubs at the package-die interface for wire bonding ease,and metal-filled vias connecting the ground plane base and the lidsealing ring to bring the lid sealing ring to RF ground.

The present invention also includes a method for designing the desiredfeedthrough that obviates the need for scale-model feedthrough designbefore printing, a prior art method that requires precision in the scalemodel. An electrical model of a feedthrough is first derived. Theelectrical model is then adjusted according to the parameters desiredfor a new, compensated feedthrough using any known method, includingsoftware such as TOUCHSTONE®. Finally, the new feedthrough is fabricatedbased upon the adjusted electrical model.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a compensated feedthrough fabricated inaccordance with the teachings of the present invention;

FIG. 2 is a cross section taken along the line II--II of the feedthroughshown in FIG. 1;

FIG. 3 is a plan view of an integrated circuit package fabricated inaccordance with the teachings of the present invention; and

FIG. 4 is an equivalent circuit for feedthrough of the type shown inFIG. 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a plan view of a preferred embodiment of the compensatedfeedthrough constructed in accordance with the present invention. Themain features of the feedthrough include the conductor under-wall width,which is reduced to the minimum that can be printed reproducibly inthick-film technology (currently about 6 mil); the brazing-metallizationwidth, which is likewise reduced to the minimum allowable; twoopen-circuited stubs set immediately prior to the interface between thefeedthrough and the die; and a plurality of metal-filled vias formed inthe ceramic to connect the ground plane base to the lid sealing ring.

As shown, the feedthrough conducts a signal between the exterior of anelectronics package (designated generally by reference numeral 2) and anintegrated circuit chip located in die cavity 4. The integrated circuitchip in this preferred embodiment is a monolithic microwave integratedcircuit (MMIC) that operates at frequencies as high as 20 GHz, althoughthe invention could conceivably be applied to electronic circuitpackages that utilize other known integrated circuits.

The present compensated feedthrough includes first microstriptransmission line section 6, transmission line section 16, and secondmicrostrip transmission line section 22. Depending upon the exact designand environment for the package, microstrip section 6 may connect to anexternal transmission line (e.g., coaxial) via pin lead 8 as shown, or,alternatively, via any input known to the art, including a surface mountinput. With reference also to FIG. 2 (a cross section of FIG. 1 lookingin the direction of arrows II--II), microstrip section 6 includesmetallization 10 forming a signal conductor on dielectric layer 12 usingknown techniques. Ground plane base 14, which is preferably acopper-tungsten alloy, provides an RF ground for the package.

Transmission line section 16 comprises metallization 10 forming a signalconductor plated between dielectric layers 12 and 18, ground plane base14, and upper sealing ring 20, which is formed over second dielectriclayer 18 using known techniques. As shown in FIG. 1, metallization 10 isnarrowed, or "necked down", when it passes below second dielectric layer18 ("under-wall"). Section 16, which resembles stripline or TRIPLATE®,is nevertheless not pure stripline since lid sealing ring 20 is not atrue RF ground. Therefore, section 16 may be referred to as apseudostrip section of transmission line, and analyzed accordingly.

Second microstrip transmission line section 22, which is similar tomicrostrip section 6, terminates in bond pad 24, formed substantiallysquare, of metallization layer 10 just before die cavity 4. Bond pad 24is well known in the art as effecting two open-circuited stubs, whichare modeled as a shunt capacitance. Bond pad 24 also enhances wirebonding to the MMIC.

In order to improve the stripline transmissive characteristics ofpseudostrip section 16 and the package as a whole, a plurality ofapertures, or vias, 26 are formed in the package. Vias 26 extend fromground plane base 14 up through the dielectric material to lid sealingring 20, and are filled with metal to bring lid sealing ring 20substantially to RF ground. When vias 26 are spaced between the variousfeedthroughs as shown in FIG. 3, they represent an improvement overprior art packages, which rely solely upon metallization of the internalwalls of the package to reference the lid sealing ring. While internalwall metallization is preferably retained in the present IC package,vias 26 both improve the status of lid sealing ring 20 as RF ground andgreatly improve isolation between leads, which could not be accomplishedsimply by metallizing the internal walls.

FIG. 3 illustrates a representative package incorporating sixfeedthroughs having the compensation described with respect to FIG. 1,with like reference numerals designating like elements. As shown in FIG.3, vias 26 are preferably spaced approximately equidistantly between thevarious feedthroughs that are located in the waveguide portion 28 of thepackage. As many or as few vias as are desired may be provided in thepackage up to the limits of manufacturing technology. The more vias, thebetter the isolation.

The feedthrough design described above is based on a low-pass filterconsisting of the lead-attachment area, themicrostrip-pseudostrip-microstrip transmission line, and the dualopen-circuited stubs. Although low-pass filters have been used in theprior art to provide compensation at the feedthrough, the presentinvention allows, within manufacturability limits, all parameters to beadjusted, and the complete filter to be printed.

These objectives are accomplished by utilizing a modeling technique todesign an appropriate feedthrough. One way to establish a desiredelectrical model is to begin with an existing feedthrough, from which atemporary equivalent electrical circuit may be determined. Theequivalent circuit is temporary in that its specific component valuesare adjustable depending upon the electrical characteristics desired forthe compensated feedthrough. Once the temporary equivalent circuit isestablished for the existing feedthrough, the values for the temporaryequivalent circuit are adjusted to achieve desired electricalcharacteristics. Working backward, the equivalent circuit that achievesthese desired electrical characteristics becomes the basis for physicalmanifestation in a new feedthrough design.

The basic structure of the feedthrough described above is well known inthe art. For example, lead attachments are known, microstriptransmission lines are known, stripline transmission lines are known,and the structures used to create all of the above are known.Fabricating the known elements, however, in the manner shown above, andto create the structure shown above, is not known to the prior art.

Referring to FIG. 4, the equivalent electrical circuit of a feedthroughsimilar to the subject feedthrough package is shown. This sameequivalent circuit applies to any such feedthrough, including both theexisting and desired feedthroughs mentioned above. Moving from left toright (with respect to both FIGS. 2 and 4):

C₁ =discontinuity capacitance at the interface between lead 8 andmetallization 10;

C₂ =discontinuity capacitance created where metallization 10 enters thepackage wall 18;

C₃ =discontinuity capacitance where the width of metallization 10narrows under the package wall 18;

C₄ =discontinuity capacitance as narrow line metallization 10 leaves thepackage wall 18;

C₅ =end-effect discontinuity capacitance created at the microstrip-bondpad 24 interface;

C₆ =bond pad 24 capacitance;

Z₁ =impedance of the lead 8 attachment area;

Z₂ =impedance of the wide line section of metallization 10 under wall18;

Z₃ =impedance of the narrow-line metallization 10 under wall 18;

Z₄ =impedance of the bond pad 24; and

L₁ =combination of end-effect inductance and wire bond inductance.

Z₂ and Z₃, while shown as block impedances, are more accuratelycharacterized by dividing them into 10 inductor/capacitor networks,since the pseudostripline cannot be treated as pure stripline, asmentioned above.

A preferred method of fabricating a desired feedthrough is to adjust thecomponent values of the equivalent circuit using a software package, forexample TOUCHSTONE®. Once the model achieves the electricalcharacteristics of a desired feedthrough, the actual feedthrough can befabricated. In the instant case, the entire low-pass filter is createdusing conventional procedures for manufacturing IC packages, with theentire low-pass filter incorporated into the package itself. In thismanner, discrete electrical components need not clutter the package, noris it necessary to include such compensation in the IC itself.

The thick-film package is preferably manufactured using two layers of94-96 percent alumina ceramic tape, metallized in the green state withrefractory-grade tungsten, laminated, and high-temperature-cofired toproduce a monolithic body. The lead frame and ground plane base areattached to the ceramic body by using high-temperature braze material.The ground plane base material is preferably Cu-W, but may also beKovar, Al₂ O₃, BeO, or AlN, for example. The package is then finishedwith nickel and gold plating suitable for wire-bonding and die-attachprocesses. Lidding is accomplished using a gold-plated Kovar lid and aeutectic Au-Sn preform.

A thin-film version of the package described above would employessentially the same physical outline. The ceramic body in the thin-filmversion, however, is manufactured from high-grade ceramic layersattached using a glass frit having a lower melting point than thatnormally used.

The thickness of the main walls of the package are thinner than in thecofired version (approximately 15 mil, as opposed to 25 mil), and thelines beneath these walls are defined at 5-mil width (compared with6-mil width in the cofired package). In all other respects, thethin-film package is structurally quite similar to the thick-filmversion, as is its performance.

Manufacture of the thin-film package differs slightly, however.Beginning with a large ceramic, the feedthroughs are printed, and theceramic fired at comparatively low temperatures. The vias are thenformed using a laser, and the die cavities cut, on a batch basis.Finally, conductors and vias are metallized and the ceramic cut intoindividual packages (assuming a batch process). After brazing the groundplane bases to the individual packages, leads may be welded on insteadof brazed, which reduces the width of the lead-conductor attachmentarea.

Various modifications of the invention discussed in the foregoingdescription will become apparent to those skilled in the art. All suchvariations that basically rely on teachings through which the inventionhas advanced the art are properly considered within the spirit and scopeof the invention.

I claim:
 1. A method for manufacturing a monolithic integrated circuitpackage having a perimeter, comprising the steps of:(a) printing atleast one feedthrough on a first surface of a ceramic substrate havingfirst and second opposed surfaces; (b) firing the ceramic; (c) forming aplurality of visa in the ceramic substrate, each said via extending fromsaid first surface to said second surface; (d) cutting a die cavity inthe ceramic for each package to be formed so that each via is located ata point distant from both said die cavity and the perimeter of each saidpackage; (e) metallizing at least one electrical conductor on theceramic substrate, each said electrical conductor corresponding to aprinted feedthrough; (f) metallizing the vias; and (g) brazing a groundplane base to the ceramic substrate on a second surface opposite saidfirst surface.
 2. A method for manufacturing a monolithic integratedcircuit package as claimed in claim 1, further comprising the step,after step (f), of:(f') cutting the ceramic substrate into individualpackages.
 3. A method for manufacturing a monolithic integrated circuitpackage as claimed in claim 1, further comprising the step of:(h)welding a lead to each metallized feedthrough.